Imaging of deep venous thrombosis using radioactive-labeled tirofiban.

The development of radiolabeled small peptide or peptidomimetic ligands can bind platelets and their specific expressed receptor have been suggested as a new approach to detect the clot location and, more essentially, to determine the age and morphology of the evolving thrombus. This new approach is focused on the use of a series of radiolabeled platelet GPIIb/IIIa receptor antagonists. Tirofiban N-(butylsulfonyl)- 4-O-(4-(4-piperidyl)-L-tyrosine is a non-peptide tyrosine derivate. The aim of the study was to introduce radioactive-labeled tirofiban as a specific imaging agent for acute DVT. The labeling was performed with technetium-99 in the presence of a stannous reducing agent. The labeled preparation showed fast blood clearance in a normal rat model (without induced thrombosis). More than 80% of the injected dose was eliminated from the circulation in the first hour after injection. Biodistribution and visualization of the labeled molecule was carried out using an experimental model of thrombosis in a male Wistar rat. Planar images were obtained 30 and 60 min after application of 2×10(6) imp/min 99m-technetium-tirofiban in the rat's tail vein. Sensitivity and specificity were determined using the ratio of 'left leg positive for DVT' to 'right leg negative for DVT'. The obtained ratio was 1.54 after 30 min and 5.04 after 60 min. These values were considered positive in the detection of acute DVT. The high DVT uptake shows that radiolabeled tirofiban in the introduced rat model can be a promising agent for imaging the deep venous thrombosis (Fig. 7, Ref. 25).

Radiolabeled bivalent haptens for tumor immunodetection and radioimmunotherapy.

The pretargeting technique referred to as the Affinity Enhancement System (AES) uses bispecific antibodies and radiolabeled bivalent haptens that bind cooperatively to target cells in vivo. Experimental and clinical data demonstrate that DTPA bivalent haptens can deliver large radiation doses to tumor cells with high tumor to normal tissue contrast ratios and long activity residence time in tumors. Preliminary clinical results of radioimmunotherapy of medullary thyroid carcinomas and lung cancers look promising. Very encouraging results in biodistribution and radioimmunotherapy experiments in animals have been obtained with new haptens bearing two histamine-hemisuccinate suitable for 131I, 99mTc and 188Re labeling. Targeting isotopes to double antigen positive tumor cells provides a binding enhancement that increases specificity for tumor cells as compared to single antigen targeting on normal cells. This approach may be beneficial for targeting isotopes to B type acute lymphoblastic leukemia and Burkitt lymphoma, as well as others tumors co-expressing two markers of low specificity, and might increase tumor irradiation with minimal irradiation of normal cells.

Bivalent hapten-bearing peptides designed for iodine-131 pretargeted radioimmunotherapy.

Pretargeting with bispecific antibodies has been used successfully for tumor detection and is now considered for radioimmunotherapy. The advantages of bivalent haptens have been demonstrated in this context. A series of bivalent molecules allowing efficient labeling with radioactive iodine has been designed for use with this new technology. They were based on the histamine-hemisuccinate hapten and prepared by solid phase peptide synthesis. Simultaneous binding of two antibody molecules to one bivalent hapten was possible with low steric hindrance when the two hapten groups were attached to the lateral chains of lysine residues separated by a single amino acid. Bispecific antibodies to the hapten and to carcinoembryonic antigen were shown to mediate specific binding of the haptens to tumor cells in vitro. These experiments demonstrated that the bivalent hapten AG3.0, with a lysyl-D-tyrosyl-lysine connecting chain, possessed the best binding properties. This peptide was used to target iodine-125 to human colon cancer xenografts in nude mice. High tumor uptake and tumor to normal tissue ratios were observed. This peptide thus appears as a good candidate for further development. Asymmetric bivalent haptens, with one histamine-hemisuccinate and one diethylenetriaminepentaacetic acid group, have also been prepared and shown to be capable of binding simultaneously two specific antibody molecules. These peptides should be useful to target radioiodine to cells characterized by the expression of two different antigenic markers.